1. Field of the Invention
The present invention relates to a welding machine controller, and more specifically, to the control elements of a welding machine controller for a resistance welding machine.
2. Description of the Prior Art
The welding mode of a resistance welding machine needs to be changed according to the quality of the workpiece. Accordingly, the welding operation of the resistance welding machine is controlled by a step control program for carrying out satisfactory welding operation meeting the quality of the workpiece. As shown in FIG. 3, controlled variables to be controlled at each step of the control program are squeeze time SQ, welding time WE, hold time HO, OFF time OF, cool time CO, welding current CU, and changeover valve control variables VC. These controlled variables are determined in consideration of the quality of the workpiece.
FIG. 4 illustrates the constitution of a conventional welding machine equipped with such a controller. Referring to FIG. 4, a resistance welding machine 1 has a welding transformer 2 and a controller 3. A squeeze time SQ, a welding time WE, a hold time HO, an OFF time OF, a cool time CO, a welding current CU, and changeover valve control variables VC for each step of the control program are given to the controller 3. The controller 3 provides control signals for controlling the intermittent supply of a welding current to the welding machine 1 and for properly controlling the supply of working fluid to the power cylinder 5 of the welding machine 1 by controlling a selector valve 4 so that the welding operation is achieved satisfactorily. Also shown in FIG. 4 are a low-pressure valve (reversing valve) 7, a high-pressure valve 8, a compressor 9, and a pressure changeover valve 10.
Pressure to be applied to a workpiece 6 is regulated according to the thickness of the workpiece 6. When the workpiece 6 has a small thickness, the changeover valve control variables VC are determined according to the small thickness, and the corresponding control signal is given through a circuit 11 to the pressure changeover valve 10 so that the pressurized fluid is supplied from the compressor 9 through the low-pressure valve 7, the pressure changeover valve 10 and the selector valve 4 to the upper chamber of the power cylinder 5. Upon the completion of the welding work, the controller 3 gives a signal through a circuit 12 to the selector valve 4 so that the working fluid is supplied to the lower chamber of the power cylinder 5 to separate the electrode of the welding machine from the workpiece 6.
When the workpiece 6 has a large thickness, the changeover valve control variable VC are determined for the large thickness, and the corresponding control signal is given through the circuit 11 to the pressure changeover valve 10 so that the pressurized working fluid is supplied from the compressor 9 through the high-pressure valve 8, the pressure changeover valve 10 and the selector valve 4 to the upper chamber of the power cylinder 5. Upon the completion of the welding work, the controller 3 gives a signal through the circuit 12 to the selector valve 4 so that the working fluid is supplied to the lower chamber of the power cylinder 5 to separate the electrode of the welding machine 1 from the workpiece 6.
Thus, the welding operation is carried out automatically according to the previously determined changeover valve control variables VC in consideration of the possible thicknesses of workpieces, and stored in the controller 3.
The pressure changeover valve 10 may be constituted so as to actuate both the low-pressure valve 7 and the high-pressure valve 8 simultaneously.
It is also possible to change the pressure working on the workpiece during the welding operation. That is, the changeover control variables VC are determined so that the pressure changeover valve 10 is controlled so as to supply the working fluid of a low pressure through the low-pressure valve 7 to the power cylinder 5 for a predetermined time at the initial stage of the welding operation and so as to supply the working fluid of a high pressure through the high-pressure valve 8 to the power cylinder 5 for a predetermined time at the final stage of the welding operation. Thus, at the initial stage of the welding operation, a signal is given to the selector valve 4 so that the pressure of the working fluid supplied from the compressor 9 is reduced to an initial pressure by the low-pressure valve 7, and then the working fluid of the initial pressure is supplied to the upper chamber of the power cylinder 5 through the selector valve 4. After a desired time has passed, another signal corresponding to the changeover valve control variables VC are given through the circuit 11 to the pressure changeover valve 10 to changeover the pressure changeover valve 10 so that the pressure of the working fluid supplied from the compressor 9 is regulated to a final pressure by the high-pressure valve 8, and then the working fluid of the final pressure is supplied through the selector valve 4 to the upper chamber of the power cylinder 5.
Thus, the welding pressure applied to the workpiece is varied in two steps, when necessary.
In the recent welding operation using a robot welding head, it is often that various workpieces are welded successively and that the thickness of the workpiece varies for weld points. Accordingly, the welding force must be varied according to the variation of the thickness of the workpieces. In order to regulate the welding force, the low-pressure valve 7 and the high-pressure valve 8 need to be adjusted. However, the adjustment of the low-pressure valve 7 and the high-pressure valve 8 requires troublesome work and, sometimes, the low-pressure valve 7 and the high-pressure valve 8 are adjusted incorrectly.
Furthermore, when the welding force needs to be regulated in steps, a pressure changeover valve must be provided in addition to a plurality of pressure regulating valves corresponding to the number of steps of welding force regulation, which requires a complicated welding machine controller as well as a large floor area for installing the same.